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Título

Microfossils (Foraminifera and Ostracoda) in deposits of the Buracica Local Stage of the Recôncavo Basin: a controversial paleoenvironmental record

Texto do resumo

The Recôncavo Basin constitutes the southernmost portion of an early Cretaceous intracontinental rift system, originated as a result of the crustal stretching leading to the Gondwana breakup. The early Barremian shales of the São Sebastião Formation have been interpreted as deposited in a lacustrine environment based on its rich non-marine ostracod fauna associated with gastropods, bivalves, ‘conchostracans’ (Diplostraca), fish and plant remains. In this context, the Marker bed B, a black shale horizon, containing Foraminifera and ostracod-bearing limestone lenses has been regarded as a geological enigma for decades due to the presence of foraminifera in an intracontinental rift supposedly before the first marine ingression. This horizon represents an important datum within the São Sebastião Formation, occurring throughout the Recôncavo Basin. Foraminifera were recovered from only the 0.125 mm size fraction of all samples, which altogether yielded 2,868 specimens belonging to three agglutinated taxa (Glomospirella arctica, Glomospirella spp. and Paratrochamminoides kaminskii). The recovered specimens are mostly poorly preserved due to compression and recrystallization and constitute a low diversity assemblage composed of small-sized agglutinated foraminifera. Ostracods occur mainly as closed carapaces in the 0.250 mm and 0.125 mm size fractions. And their assemblages indicate a lacustrine, freshwater to oligohaline environment. The occurrence of the ostracod species Petrobrasia glabra, P. tenuistriata tenuistriata and P. tenuistriata longiuscula indicates an upper Hauterivian? to lower Barremian age (Buracica Local Stage) for the studied samples. Specimens belonging to the genera Alicenula, Cypridea and Theriosynoecum were also identified. Cypridea specimens with nodes were identified in one of the wells analyzed, at the base of the studied interval, where the number of Foraminifera is reduced in relation to the samples in the section above. The presence of node in non-marine ostracods is presumed to be associated with frequent pulses of strong freshwater inflow into (low) saline waters, which may justify the reduced number of foraminifers at this level. The low diversity of foraminiferal associations with small specimens indicates limiting conditions for the establishment of a diverse fauna. Associations of agglutinated foraminifera with low diversity and reduced specimen size generally reflect high reproduction rates, which is commonly related to opportunistic forms, that thrive under conditions being adverse for other species. No other records of marine organisms other than foraminifers were found. Therefore, due to the lack of evidence for a shallow marine paleoenvironment we suggest an athalassic saline paleoenvironment into which foraminifera could be carried by migratory flying vertebrates as it was reported in several studies in (paleo)lakes deposits.

Palavras Chave

Recôncavo Basin; Foraminifera; Ostracoda; Continental rift; paleoenvironment